The present invention relates to thermoformable elements, of the kind comprising two adjacent layers of polymers at least one of which comprises an elastically compressible foam, at least one of the layers being made of a thermoplastic polymer, and to a use of such element.
The use of thermoplastic polymer foams, in particular polyethylene, has already been proposed in the medical field, particularly as an orthopaedic support for immobilising fractured limbs. To this end, the foam is heat-moulded around the part of the body to be bandaged. Foams of this type have also been used to fit a prosthesis to an amputated limb. To this end, the foam is heated to its thermoforming temperature which is approximately between 130.degree. and 140.degree. C. and is applied to the part of the body to which it is to be fitted, by modelling it until it takes the form of that part of the body.
Other applications for the foam have been proposed, in particular in European Patent No. A 2 0 004 829, in which the foam is associated with an electric heating body in order to enable moulding of the foam, a rough cast of which has been made previously for fitting closely a non-extensible, in particular rigid, casting, to a part of the human body.
Although the association of an electric heating body is possible in the case of an industrial item which, by definition, is mass produced with the same dimensions, this solution is difficult to adapt for medical use, in particular orthopaedic use, in that each bandage not only has to correspond to the shape and size of the patient, but also to the fracture or fractures, as well as to the location thereof. In view of these constraints, it would be inconceivable, in practice, to make electric heating bodies dimensioned according to the size of each bandage, since this solution would lead to a multiplicity of heating bodies, and to form them in such a way as to produce homogeneous heating across the entire surface of the bandage, which is extremely difficult to carry out and has to be studied from case to case. This condition is of great importance for thermoforming since foams of this type have very poor thermal conductivity, so that the density of current per section of the heating body has to be as constant as possible for each portion of the foam surface to be thermoformed.
For this reason it has been proposed to make orthopaedic supports using thermoformable materials without incorporating heating means therein. The orthopaedist cuts pieces of thermoformable materials to the required dimensions or buys elements which are sold ready cut, heats them in an oven or in a hot water bath according to the thermoforming temperature of the material used and then forms it on the patient. An orthopaedic bandage of this type is described in French Patent No. 1 570 760. In this case, a sheet of a thermoplastic material which is rigid at room temperature is inserted between two sheets of a foam made of thermoplastic material, in order to provide support for the limb to be immobilised.
In French Patent No. 2 120 515 a splint has also been proposed which comprises a rigid shell the inner face of which is lined with a type of impervious cavity formed by a flexible casing. A fluid or material which can be hardened can be injected into the cavity in order to immobilise the limb inside the rigid shell. A solution of this type is difficult to produce on an industrial scale in that different sizes of shell have to be produced, the shell having unchangeable dimensions.
Another factor which should be considered in thermoforming is the compatibility of the thermoforming temperature with what the skin can withstand. In the case of flexible foams, such as those used for example in French Patent No. 1 570 760, their low density makes it possible to withstand a relatively high thermoforming temperature. On the other hand, in the case of denser materials, for example materials which are rigid at room temperature, a temperature of 50.degree. to 60.degree. C. cannot be exceeded if the technique of heating in an oven is used and if the materials are to be applied around the limb to be immobilised whilst they are still soft. This has made necessary the development of special materials for this application which are very expensive, so that their use is restricted as a result.
The object of the present invention is to provide a thermoformable element which ensures even heating of the entire surface of the foam to be thermoformed, whatever the shape or size of the surface.